The present invention relates to a thermal protector to be mounted in a current circuit between an electric load and a power supply, in which a bimetal connects or disconnects the current circuit, depending on thermal conditions.
In a motor or a product using a motor-compressor such as a refrigerator, an air conditioner, a dehumidifier, etc. generally, a thermal protector is provided in order to prevent burning due to overheating of the motor. A thermal protector is also provided for preventing overheating of a heater, etc. Previously various types of thermal protectors have been proposed. As an example thereof those disclosed in JU-A-59-72641, JU-A-64-35642, etc. will be explained, referring to FIGS. 1 and 2.
The thermal protector of FIGS. 1 and 2 includes a case 1 having an outer bottom surface 1a, an inner bottom surface 1b, a cover 2, movable contacts 3, 4, a bimetal 5, an adjusting screw 6, a head portion 6a on the screw 6, fixed contacts 7, 8, fixed terminals 9, 10, a heater terminal 11, a heater wire 12, and a coil spring 13.
The case 1 is made of a heat-resisting and insulating material such as a synthetic resin, etc., e.g. phenol resin or unsaturated polyester, having a cylindrical form with a bottom. This case 1 is covered with the cover 2 so that an internal space is formed.
In this internal space, the adjusting screw 6, made of brass, is mounted at the center of the bottom, passing therethrough from the inner bottom surface 1b to the outer bottom surface 1a and the head portion 6a is formed at the end of this adjusting screw 6 inside of the case 1. A disk-shaped bimetal 5 is mounted on this adjusting screw 6. Further, the spring 13 is mounted thereon between the bimetal 5 and the inner bottom surface 1b of the case 1. The bimetal 5 is pressed against the head portion 6a of this adjusting screw 6 by energizing force of this spring 13.
The two movable contacts are secured to the end portions of the bimetal 5 on the surface facing to the inner bottom surface 1b of the case 1. Further, the fixed contact 7 at the extremity of the fixed terminal 9 secured to the case 1 so as to pass therethrough from the inner bottom surface 1b to the outer bottom surface 1a is fixed in opposition to the movable contact 3. In the same way the fixed contact 8, at the extremity of the fixed terminal 10 secured to the case 1 so that a part thereof protrudes outward, is fixed in opposition to the movable contact 4. The heater wire 12 is connected between the heater terminal 11 and the fixed terminal 9 protruding similarly outward by welding, etc. The fixed terminal 10 and the heater terminal 11 serve as external terminals for the thermal protector. This heater wire 12 is disposed so as to be close to the lower surface of the bimetal 5 and to pass the farther side with respect to the adjusting screw 6. The bimetal 5 is heated over the entire periphery by heat produced by the heater wire 12.
The bimetal 5 is shaped in a curved form around the central portion thereof. When the temperature is less than a set value, it is shaped in a curved form, in which the central portion thereof protrudes upward, so that the movable contacts 3 and 4 are in contact with the fixed contacts 7 and 8, respectively. In this way a current path is formed from the fixed terminal 10 to the heater terminal 11 through the fixed contact 8, the movable contact 4, the bimetal 5, the movable contact 3, the fixed contact 7, the fixed terminal 9 and the heater wire 12. When the temperature rises and arrives at the predetermined temperature, the bimetal 5 is rapidly deformed into a curved form, in which the central portion protrudes in a downward direction, contrarily to that indicated in FIG. 1. Hereinbelow this is called inversion movement and the state of the bimetal 5 after the inversion movement is called inverted state. Further the temperature, at which this inversion movement takes place, is called inversion movement temperature. When the bimetal 5 effects the inversion movement, the movable contacts 3 and 4 are separated from the fixed contacts 7 and 8, respectively, so that the current path is in a cut-off state.
When the bimetal 5 is in the inverted state and the temperature is lowered down to the predetermined temperature, the bimetal 5 returns to the state indicated in FIG. 1. Hereinbelow this is called return movement and the state indicated in the figure is called initial state. Further the temperature, at which the return movement takes place, is called return movement temperature. When the bimetal 5 returns from the inverted state to the initial state, the movable contacts 3 and 4 are once again brought into contact with the fixed contacts 7 and 8, respectively, so that the current path is reformed.
In the winding circuit of a motor 15 in FIG. 3, a thermal protector 14' is provided, with the winding circuit including a starting device 16, a starting winding 17, and a main winding 18.
In FIG. 3, only the current path portion described above is indicated for the thermal protector 14' and only the winding portion is indicated for the motor 15. In the motor 15, a series circuit of the starting winding 17 and the starting device 16 is connected with the main winding 18 in parallel. The motor 15 and the thermal protector 14' are connected in series by connecting one of the terminals of the motor 15 with the heater terminal 11. In this manner, current flows from the power supply 20 to the starting winding 17 of the motor 15 and the main winding 18 through the fixed terminal 10, the bimetal 5, the heater wire 12 and the heater terminal 11.
When mechanical lock of the motor is produced during drive of the motor due to burning of bearing portions of the motor 15 or the compressor driven by the motor 15 or penetration of dust into rotating parts, a high intensity current corresponding to the starting current continues to flow therethrough, because the rotor does not rotate. This high intensity current continues to flow, as far as the rotor is locked in the state where the power supply is connected therewith. This current is called lock current. The lock current is about four to five times as high as the nominal current of the motor 15. The duration of the starting current (starting period) at normal start is usually as short as two to three seconds and the motor is so designed to bear satisfactorily such a high intensity starting current flowing in this short period of time. However, in the design, it is not taken into account that the lock current continues to flow for a long period of time through the motor 15 and the current circuit therefor, which is not desirable.
When a large current flows through the motor 15, heat generation by the bimetal 5 and the heater wire 12 increases. At the moment where the temperature of the bimetal 5 arrives at the inversion movement temperature, the bimetal rapidly effect the inversion movement. Thus, the movable contacts 3 and 4 are separated from the fixed contacts 7 and 8 so that feed of current to the motor 15 is stopped. After the feed of current to the motor 15 has been stopped, the bimetal 5 and the heater wire 12 begin to be cooled. When the temperature is lowered down to the return movement temperature of the bimetal 5, the bimetal 5 rapidly effects the return movement to return to its initial state. Thus, the movable contacts 3 and 4 are brought into contact with the fixed contacts 7 and 8, respectively, so that the feed of current to the motor 15 is again started.
At this time, if the locked state of the motor 15 is removed, the bimetal 5 does not effect again the inversion movement and thus the motor 15 is driven normally.
Next another example of a prior art thermal protector of the type described, for example, in JU-A-60-183349 will be explained, referring to FIG. 4.
This prior art example differs basically from the prior art example indicated in FIG. 1 in that no heater wire is provided. For this reason, the fixed terminal 9 having the fixed contact 7 at the extremity passe through the bottom portion of the case 1 so as to protrude outward and constitutes external terminals together with the fixed terminal 10. When the movable contacts 3 and 4 contact the fixed contacts 7 and 8, respectively, a current path is formed from the fixed terminal 10 to the fixed terminal 9 through the fixed contact 8, the movable contact 4, the bimetal 5, the movable contact 3 and the fixed contact 7.
When such a thermal protector 14" is used for the motor 15, one of the fixed terminals 9 of the thermal protector 14" is connected with one of the terminals of the motor 15 (FIG. 5).
When some abnormalities are produced in the motor 15 and a high intensity lock current flows therethrough, heat generation by the bimetal 5 increases. At the moment where the temperature is raised up to the inversion movement temperature of the bimetal 5, the bimetal rapidly effects the inversion movement, the movable contacts 3 and 4 are separated from the fixed contacts 7 and 8, respectively, and feed of current to the motor 15 is stopped. When the feed of current is stopped, the bimetal 5 begins to be cooled. When the temperature is lowered to the return movement temperature of the bimetal 5, the bimetal 5 rapidly effects the return movement to return to its initial state. Thus the movable contacts 3 and 4 are brought into contact with the fixed contacts 7 and 8, respectively, and the feed of current to the motor 15 is restarted started again.
At this time, if the locked state of the motor 15 is removed, the bimetal 5 does not effect return movement anymore and the motor 15 is driven normally.
As described above, according to each of the prior art examples described above, if the locked state is removed during the inverted state of the bimetal 5, the motor 15 returns to the normal drive state and thus burning due to overheating can be prevented.
However, if the abnormal state of the motor 15 is not removed, the bimetal 5 effects the return movement to return to its initial state. But, if the motor 5 is again in the locked state, a high intensity lock current flows through the thermal protector 14" and the bimetal 5 again effects the return movement. Thus, the bimetal 5 is in the locked state and the feed of current to the motor 15 is stopped.
As described above, if the abnormal state of the motor 15 is not removed, the bimetal 5 repeats the inversion movement and the return movement and after a number of these repetitions, the bimetal 5 is finally fatigued and broken.
In JU-A-60-183349 cited previously, as shown in FIG. 6 a bimetal 5 is used, in which stress dispersing slits 5c are disposed radially around a shaft supporting hole 5b, in which the adjusting screw is inserted. If such a bimetal 5 repeats the inversion movement and the return movement, breaks extending from extremities of the slits 5c towards the outer periphery 5d, as indicated by E and F, are produced. As the result of the breaks, the radius of the bimetal 5 is increased by internal stress in the bimetal 5 and the outer peripheral portions 5d in the neighborhood of the contacts are deformed from those indicated by chain-dotted line to those indicated by full lines, as indicated in FIG. 6.
If the bimetal 5 is broken, characteristics of the bimetal 5 are changed. Thus, the inversion movement temperature and the return movement temperature are changed or even if it effects the inversion movement, the amount of the inversion movement is decreased. In this way the interval between inversion movements is shortened and the rate of the lock current flowing through the bimetal 5 and the heater wire 12 increases. Therefore, the temperature within the case rises more and more. Further, this is accompanied by decrease in the contact pressure of the movable contacts 3 and 4. If the lock current continues to flow, the movable contacts 3 and 4 are finally bonded by fusion with the fixed contacts 7 and 8, respectively. When bonding by fusion of the contacts is produced as described above, the high intensity lock current flows without interruption through the winding of the motor 15 and the bimetal 5 in the thermal protector 14' or 14" and the winding of the motor 15 generates heat, which gives rise to burning. Further, the temperature within the case 1 is raised by heat produced by the bimetal 5 and the heat wire 12 and when the temperature exceeds the tolerable highest temperature of the case 1 and the cover 2, the case 1 and the cover 2 are burned in the neighborhood of the bimetal 5.
In the prior art example indicated in FIG. 1, if the heater wire 12 is broken when the temperature within the case 1 is increased, the current path in the thermal protector 14' is cut and therefore the burning described above can be prevented. Thus, there is no danger. However, the heater wire 12 is not always broken. Further in the thermal protector having no heater wire indicated in FIG. 4 even this operation cannot be expected.
Heretofore various means have been proposed for solving such problems as described above.
As one of them, e.g. in JU-A-59-72641, a thermal protector using heat resisting materials such as ceramics for the case is disclosed.
Furthermore, e.g. in JU-A-63-174145, means is disclosed, in which there is disposed a movement counting plate having a plurality of sawtooth-shaped protrusions, which are engaged one after another with the bimetal every time it effects a return movement, to lower the movement counting plate. When the bimetal effects the return movement a number of times, which is equal to the number of the sawtooth-shaped protrusions, the movement counting plate is brought into contact with the inner bottom surface of the case so that the bimetal cannot effect the return movement anymore. According thereto, even if the abnormal state of the motor is not removed, if the bimetal has effected the return movement a predetermined number of times, it cannot effect the return movement anymore. Therefore, the inverted state is held and the lock current is cut off.
Still further, in, for example, JU-A-63-224125, means is disclosed, in which there is disposed a second bimetal having an inversion movement temperature higher than that of a first bimetal connected in series with the second bimetal, the inversion movement of the first bimetal is produced by generation of abnormal current, the abnormal state being not removed, the first bimetal repeats the inversion movement and the return movement, and when the first bimetal is finally destroyed and the contacts are bonded by fusion, the inversion movement of the second bimetal is produced by abnormal rise of the temperature taking place as the result, which cuts off the abnormal current.
Still further, in JU-A-64-1450 or JP-A-1-82424, a technique is disclosed, by which a second bimetal is brought into contact with the lower surface of a first bimetal and when the first bimetal is broken and contacts are bonded by fusion, the second bimetal effects the inversion movement to raise the first bimetal.
Still further, in JU-A-64-35642 or JP-A-1-279532 and JP-A-1-44232, a technique is disclosed, by which the head portion of the shaft, on which the bimetal is mounted, is a part separated from the shaft, a recess is formed in this head portion, and this recess is filled with thermally fusible metal, when this head portion is engaged with the shaft, so that the head portion is secured to the extremity of the shaft by this thermally fusible metal. Normally, the bimetal is pressed against the head portion by a spring, but when the contacts of the bimetal are bonded by fusion and the temperature is raised, the thermally fusible metal is melted so that the sticking between the head portion and the shaft is removed and the bimetal and the head portion are raised by energizing force of the spring.
Still further, in, for example, JU-A-2-128338, a technique is disclosed, by which an engaging metal piece is engaged with the shaft, which serves as a shaft supporting portion, and when the temperature rises, the engaging metal piece is deformed so that the engagement with the shaft is removed and the bimetal is raised by energizing force of the spring.
Furthermore, in, for example, JU-A-2-128339, a technique is disclosed, by which there is disposed a receiving metal piece o the upper surface of the bimetal to constitute a bimetal shaft supporting portion, which piece is deformed at a high temperature so as to have a small height, and the bimetal is raised by energizing force of the spring.
Although various sorts of countermeasures against bonding by fusion of the contacts of the bimetal have been proposed as described above, these have different problems as follows.
That is, when made of ceramics as described in JU-A-59-72641, there is a problem that, although burning of the case is certainly avoided, burning of the motor coil cannot be avoided and the case is expensive.
Further, the technique, by which the movement counting plate is disposed as disclosed in JU-A-63-174145, had problems as follows, because the number of repetitions of the inversion movement and the return movement is limited by this movement counting plate;
(1) For the thermal protector used in a refrigerator, an air conditioner, a dehumidifier, etc., it took place easily that the bimetal is moved even by a defect of a compressor motor or a fan apart from mechanical lock of the motor and that the bimetal is held by the movement counting plate in the inverted state, which increases the number of service calls.
(2) Such problems at realizing it in practice remain that the movement counting plate is displaced also at confirming the operation during adjusting work and that the remaining number of movements decreases.
Further, in the case where a first and a second bimetal, connected in series, were used as described in JU-A-63-224125, since it was necessary to make a current flow therethrough simultaneously, problems at realizing it in practice remained as follows:
(1) The region of the intensity of current, which can be made flow therethrough, is limited, depending on specific resistance of these bimetals.
(2) When the specific resistance of the bimetals is insufficient so that the amount of heat generated by themselves is too small, it is necessary to provide a heater wire. Since it is necessary, therefore, to secure a sufficient insulating distance between the bimetals and the heater wire, the space occupied by the heater wire is great and the size of the thermal protector increases.
(3) It is necessary to dispose expensive contacts on each of the first and the second bimetal and therefore the thermal protector itself is expensive.
Still further, in the case where the head portion is stuck to the shaft with the thermally fusible metal as described in JU-A-64-35642 or JP-A-1-279532 and JU-A-2-44232, the thermal protector has the following problems:
(1) When the contacts of the bimetal are bonded by fusion, the thermally fusible metal begins to melt and the bimetal and the head portion of the shaft are raised by the spring. However, they are only slowly raised by the spring only slowly because of viscosity of the thermally fusible metal. When the movable contacts are separated from the fixed contacts disposed on the inner bottom surface of the case due to the raising of the bimetal the current path is cut. Therefore, the feed of current to the heater is stopped and, at the same time, the heat source is lost. Thus, the state of the thermally fusible metal moves towards solid phase.
As described above, when there is no sufficient spring force for satisfactorily overcoming the viscosity of the thermally fusible metal acting thereon, no satisfactory contact separation distance (contact gap) between the movable contact and the fixed contact is secured, when the bimetal is raised.
(2) The solidification phenomenon of the thermally fusible metal described above is load resistance itself of the spring and acts so as to reduce the force for separating the contacts of the spring from each other when they are bonded by fusion. It can be presumed that this can be hindrance to obtaining a thermal protector for turning a load on and off at a high intensity current.
(3) Since there is creep in the bond by the thermally fusible metal, the melting point thereof should have a sufficiently great temperature difference with respect to the inversion movement temperature. For this reason, working temperature for performing a contact separating operation is high and utilization field of the thermal protector is apt to be limited.
(4) Equipment having a high stability is required for penetration of the thermally fusible metal into the recess formed in the head portion of the shaft and cost of equipment is therefore high.
Still further, in the case where an engaging metal piece is used at the bimetal shaft supporting portion as described in JU-A-2-128338, following problems arise:
(1) It is necessary to form a ring-shaped groove on the shaft, which causes cost-up by increase in the number of fabrication steps.
(2) Since a shape memory alloy is used for the engaging metal piece, the deformation speed is low. Therefore, it requires a long time, before it is separated from the ring-shaped groove.
(3) Since an expensive material is used for the engaging metal piece, the thermal protector itself is expensive.
Still further, in the case where a receiving metal piece is used on the upper surface of the bimetal as described in JU-A-2-128339, there are the following problems:
(1) When the threaded portion of the shaft is rotated to adjust the inversion movement temperature to a predetermined set temperature and at the same time to obtain a contact pressure between the movable contact and the fixed pressure, the shaft supporting position for the bimetal is moved and at the same time bending magnitude of the receiving metal piece is varied. Accompanied by these variations, working temperature of the receiving metal piece is varied also.
(2) Further the shaft supporting position for the bimetal at the predetermined set temperature is determined at a position where the resultant force of the resilient force of the bimetal and the energizing force of the spring and the force of the receiving metal piece are balanced. The point where the two forces are balanced varies without interruption and, therefore, it is difficult to adjust it.
(3) In the state where the bimetal has effected the inversion movement and the movable contacts are separated from the fixed contacts, the shaft supporting position for the bimetal is determined at a position where the downward force of the receiving metal piece and the upward energizing force of the receiving metal piece are balanced.
That is, since there is no resilient force of the bimetal, the spring is varied in the compression direction by the energizing force of the receiving metal force and acts in the direction where the contact separation distance (contact gap) decreases. In the worst case, no satisfactory contact separation distance can be obtained and it is feared that it looses the function as the thermal protector.
(4) In any case, it is difficult to fabricate the thermal protector, unless size precision, etc. of different constituent parts are remarkably increased.
What can be said in common for all the prior art examples is as follows.
(1) The object can be achieved by adding some constituent parts thereto.
(2) The fabrication steps are complicated and new equipments should be added to a conventional equipment. At the same time the prime cost is remarkably increased by lowering in the fabrication yield at the adjustment, etc.